Scientists ‘untangle’ quantum communications

Entanglement is one of the foundations of quantum communications, since that’s the property that makes quantum communication schemes secure*. So why is a group of scientists from Japan and the UK proposing to work without entanglement?

A study in Nature Photonics (abstract) led by Bill Munro – a research scientist with NTT’s Basic Research Laboratories in Japan – proposes just that: an entanglement-free quantum communications system that can transmit 107 quantum states per second.

The reason, Monro explained to The Register, is this: although entanglement leads to security, that’s not the only possible application of quantum communications. A quantum channel can also be used to communicate between quantum devices inside a quantum computer – in which case, speed is rather more important than security.

He also notes that achieving acceptable speed over long distance is a key focus of quantum communications research. Relatively slow communications in the terabit age is acceptable if the quantum channel is only being used for key distribution (with the key then used to encrypt a conventional communications channel), but not if you want to replace the classical channel entirely.

“What we are trying to do is to avoid having to create entanglement between repeater nodes (as this is slow and requires classical messaging)”, he told The Register.

These slow speeds are especially problematic when communicating over a long distance, something that requires repeaters, since an entanglement would have to be created between each pair of nodes on the network.

“For long distance quantum communication the potential rates of communication have been quite low (kHz or lower - with the limitation being caused by the classical communication time between nodes). Hence our scheme may be able to increase the communication rates,” Monro explained.

Security could still be added over the top, between the sender and receiver instead of at each hop, but the “no-entanglement” end-to-end link would reach further and at much greater speed than quantum communication schemes that rely on entanglement.

Even though there’s no entanglement, Munro said it’s clear that the scheme is transmitting superpositions (rather than, in a conventional optical network, using the frequency of light to signify the bit that’s being sent).

Munro’s scheme encodes states onto “matter qubits” (for example, using electron spin) at the originating and destination nodes. This state is transferred at the transmitter to a photon qubit, which is transmitted down a fibre to a receiver that reverses the process.

If the matter qubits needed to be entangled, they would have to be retained until the receiver process was complete, so the two ends could “compare notes” to determine that the photon’s state was preserved in the channel. Instead, the Munro scheme proposes to use a quantum parity code to check the received state – something the paper claims allows the system to tolerate photon losses in the channel as high as 50 percent (hence extending the range of the system to 17 km between nodes).

The paper states that 48 nodes would provide an 800 km quantum link that has a 98 percent probability of successfully transmitting the quantum information. The intermediate nodes in the network could also act as routers. ®

*Bootnote:El Reg is aware that perfect "quantum communications" security is a highly qualified and controversial topic.